Packet data networks, including wired networks and/or wireless networks, are used to link various types of network devices, such as personal computers, network telephones, mobile telephones, personal digital assistants (PDAs), and so forth. A widely used type of packet data network is the Internet Protocol (IP) network, in which data communications are performed using packets or datagrams.
With the increased capacity and reliability of packet data networks, voice communications (including telephone calls, video conferencing, and so forth) over such packet data networks have been implemented. Voice communications over packet data networks are unlike voice communications in a conventional circuit-switched network (such as a public switched telephone network), in which users are provided dedicated end-to-end circuit connections for the duration of each call. In a packet data network, voice data is carried in packets or datagrams that are sent in bursts from a source to one or more destination nodes. Voice data that is sent over a packet data network typically shares network bandwidth with conventional non-voice data, such as data associated with electronic mail, web access, file transfer, text chat sessions, and so forth.
Various standards have been proposed for establishing voice and multimedia communications over packet data networks. One example standard that defines control signaling used for establishing voice and multimedia communications is the Session Initiation Protocol (SIP), which defines messaging for establishing, controlling, and terminating multimedia sessions over a packet data network, such as an IP network. SIP is part of a multimedia data and control architecture developed by the Internet Engineering Task Force (IETF). In packet-switched wireless networks, the Third Generation Partnership Project (3GPP) and 3GPP2 have defined standards for SIP call flows. Other organizations have also defined SIP call flows for use in wired and/or wireless networks.
SIP is a text-based protocol that defines SIP messages having a text format, which tends to make SIP messages relatively large in size. As a result, the increased time involved in communicating SIP messages may cause call setup times to become longer. In addition to larger SIP message sizes, another cause of relatively long call setup times is that more SIP messages are involved in establishing a call session (particularly when extra messages are sent to provide reliability) than has been traditionally the case in circuit-switched networks.
As a result of a relatively long call setup time, there may be excessive delay between when a caller starts a call (such as by activating the “Send” button on a phone or completion of dialing digits) and when the caller receives an indication of ringing (ringback that indicates that the called party is being alerted). The interval between the time a caller starts a call and the time when ringback is received by the caller is referred to as post-dial delay (PDD). Excessive PDD can cause user dissatisfaction. In some cases, a user may simply hang up if there is excessive PDD, since the user may incorrectly believe that the call has been dropped when in fact call establishment is proceeding in the packet data network among various nodes.